CMP process metrology test structures

Abstract

A method for forming metrology structures for a CMP process is described. A trench edge is formed in a base material or stack of materials which are preferably deposited as part of the process of fabricating the production structures on the wafer. A covering film of a second material with preferably with contrasting SEM properties is deposited over the trench edge in the base material. During CMP the covering film is preferentially worn away at the edge revealing the base material. The width of the base material which has been revealed is a measure of the progress of the CMP. Since the base material and the covering material are preferably selected to have contrasting images in an SEM, a CD-SEM can be used to precisely measure the CMP progress.

1. A method of fabricating thin film metrology test devices on a wafer comprising the steps of:
 depositing a base layer structure for a metrology test site; 
 forming an edge in the base layer structure by removing a selected area of the base layer structure; 
 depositing a covering material over the edge in the base layer structure; 
 performing a chemical-mechanical polishing (CMP) on the wafer; and 
 measuring a width of the base layer structure exposed through the covering material as a metric of CMP progress. 
 
     
     
       2. The method of  claim 1  wherein the base layer structure includes a layer of metal and the covering material is carbon. 
     
     
       3. The method of  claim 1  wherein the base layer structure is a layer stack for a magnetic sensor. 
     
     
       4. The method of  claim 3  wherein the step of forming an edge in the base layer structure is performed as a part of a process of milling through the base layer structure to expose alignment marks under the base layer structure. 
     
     
       5. The method of  claim 3  wherein the covering material is carbon which is deposited over the layer stack for a magnetic sensor. 
     
     
       6. A method of fabricating magnetic sensors on a wafer comprising the steps of:
 depositing a layer stack for a magnetic sensor on the wafer; 
 patterning a first mask on the wafer including an opening for at least one test site; 
 milling a trench in the layer stack for the magnetic sensor through the opening for the test site; 
 removing the first mask; 
 depositing a layer of nonmetallic material over the wafer including the test site; 
 patterning a second mask on the wafer for performing a milling process to define an edge of the magnetic sensor, the second mask covering the test site; 
 milling to define an edge of the magnetic sensor through the second mask; 
 performing a chemical-mechanical polishing to remove the second mask; and 
 measuring an edge of the layer stack at the test site exposed through the nonmetallic material to determine progress of the chemical-mechanical polishing. 
 
     
     
       7. The method of  claim 6  wherein the nonmetallic material is carbon. 
     
     
       8. The method of  claim 6  wherein the step of milling the trench is performed simultaneously with milling to expose alignment marks under the layer stack for the magnetic sensor.